The scientific argument for extraterrestrial life can be traced back to the 1960's, when astronomer Frank Drake first formulated what has come to be known as the "Drake Equation". Essentially, this is just a simple method to estimate the number of intelligent civilizations in our Galaxy. It starts with the number of stars (a very big number) and then multiplies by reasonable estimates of the fraction of stars that have planets, the fraction of those that are habitable, that form life, where the life becomes intelligent, that develop the technology to emit signals into space (like radio communication), and finally how long those civilizations exist before becoming extinct or destroying themselves. The main conclusion is that even if you make very pessimistic assumptions for all of these fractions, the number of stars is so large that there will still be a handful of intelligent civilizations like ours in the Galaxy. In Hawking's words, "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational."

It was just 15 years ago that the first planet outside of our solar system was discovered around a star like the Sun. As of this week, more than 450 "exoplanets" have been discovered around other stars. Many of these planets do not resemble those in our own solar system at all. The methods that have been used to discover them are biased towards large planets like Jupiter that orbit relatively close to their suns like Mercury (and in most cases even closer), but a few of the known exoplanet systems include up to five planets. The diversity of planetary systems that astronomers are finding around other stars suggests that planets are much more common than we originally thought, and it's a first step towards actually measuring the fractions in Drake's equation.

Despite our progress in detecting planets around other stars, we still don't know of any habitable planets other than the Earth. However, within the next few years NASA's Kepler mission will tell us exactly how common habitable planets are in our Galaxy. Launched just over a year ago, the Kepler space telescope is performing a detailed census of planets around stars in our Galactic neighborhood. Its enormous digital camera has the sensitivity to detect the tiny eclipse of a planet the size of the Earth passing in front of its host star, and the mission is monitoring more than 150,000 stars like the Sun for such signals. Since a habitable planet like the Earth takes a full year to complete an orbit, Kepler needs to search for several of these "transits" over the course of a few years to identify the distant Earths reliably. If such planets are as common as we believe, Kepler should find a dozen or so within the first few years -- but in any case, it will measure another fraction in Drake's equation.

Throughout human history whenever we have believed ourselves to be special in some way, we turned out to be wrong. We now know that the Earth is just one planet among many in our Galaxy, and we will soon know how common planets are that might also be habitable. With a short list of distant Earths, we can begin to search for alien radio signals. It may take some time, but the conclusion is inevitable -- we are almost certainly not alone.